Large Diatom Species Research Articles

Salinity-Dependent Species Richness of Bacillariophyta in Hypersaline Environments

Hypersaline habitats are among the most polyextreme habitats on Earth, but they contain a rather large diatom species diversity. A review of the diatom species’ richness was made on three scales: 1. a separate lake in Crimea; 2. all hypersaline waters in Crimea; and 3. the world totality as a whole. In total, 51 species were found in Lake Chersonesskoye during sampling from 2004 to 2018. In ten Crimean hypersaline lakes, 91 species were noted in total. All diatom species found in the studied hypersaline water bodies of the world belong to 458 species, which is 2.7% of the total number of known species of Bacillariophyta. In all three scales, the similarity of the species’ composition between water bodies as well as studied periods was not found. Most of all the identified species were found only in one of the studied water bodies, and only 59 species (13% of the total list) were found in more than three water bodies. An analysis showed that no more than 40% of the species that exist in hypersaline waters have been identified on a global scale. The diatom shell nanostructure from hypersaline waters has specific peculiarities that may be valuable for some technological applications. The knowledge of diatoms in hypersaline waters not only has pure scientific importance but also covers some needs of bio- and nanotechnologies.

Phytoplankton Growth Rate and Microzooplankton Grazing under Conditions of Climatic Changes and Anthropogenic Pollution in the Coastal Waters of the Black Sea (Sevastopol Region)

In the coastal waters of the Black Sea near Sevastopol, a gradual temperature elevation and an increase in anthropogenic pressure since the early 2000s have caused significant structural and functional changes in phytoplankton. Currently, there is a significant decrease in the contribution of small diatom species (Skeletonema sp. and Chaetoceros socialis H.S.Lauder as well as coccolithophorids Emiliania huxleyi (Lohmann) W.W.Hay and H.P.Mohler) to the total phytoplankton biomass in these waters. Previously these species caused regular weak blooms. In the warm periods (from May to October), during which the main phytoplankton biomass is formed, large diatom species Pseudosolenia calcar-avis (Schultze) B.G.Sundström, 1986, Proboscia alata (Brightwell) Sundström and dinoflagellates predominate. Therefore, the maximum values of the phytoplankton community’s specific growth rate are about two times lower than in the preceding periods and do not exceed 1.10–1.40 day−1. There was also a decrease observed in the microzooplankton grazing rate, which, during the year, was no higher than 0.70–1.20 day−1. This is primarily conditioned by the increased role of large algae in phytoplankton, which means a decline in nutrition quality for microzooplankton. As a result of the joint influence of nutrition quality and water pollution, the relative share of net primary production consumed by microzooplankton in the warm periods of the year averaged only 32%, which is two times lower than the average values generally accepted for marine ecosystems. This means that the transfer of matter and energy from phytoplankton to higher trophic levels is significantly decreased.

The effect of diatoms on the grain size of lake sediments: a case study of the sediments of Lake Kanas

The grain size of lake sediments has been widely used to reconstruct environmental processes. However, the various components of lake sediments are influenced by different factors, and therefore the environmental significance of grain-size variations may be complex. For example, in situ authigenic biological siliceous components cannot be used to indicate variations in hydrodynamic conditions. Here we present a case study of the effects of diatoms on the grain size of the sediments of Lake Kanas in NW China. The analyses consist of measurements of bulk sediment grain size and biogenic silica, together with microscope observations. Based on both light and scanning electronic microscope observations, two large diatom species, Campylodiscus hibernicus Ehrenberg and Ellerbeckia arenaria (Moore) Crawford, were found to be abundant in sediment samples with a large bulk-sediment grain size, and they contributed substantially to the biogenic silica content and the measured bulk sediment grain size of the core. A quantitative assessment of the effects on grain size was made, and the study has a broad significance for evaluating the effects of diatoms on sediment grain-size distributions in other lakes. Because diatoms may deposit in situ or could be transported differently from other clastic material, the explanation of hydrodynamics could be misled if the influence of diatom has not been excluded. Therefore, we suggest BSi content should be tested to assess the diatom effect prior to grain-size analysis of lake sediments, together with microscope check of diatoms if BSi content is higher than 10%.

Evidence of small‐scale spatial structuring of phytoplankton alpha‐ and beta‐diversity in the open ocean

SummaryPhytoplankton assemblages in the open ocean are usually assumed to be mixed on local scales unless large semi‐permanent density discontinuities separating water masses are present. Recent modelling studies have, however, suggested that ephemeral submesoscale oceanographic features leading to only subtle density discontinuities may be important for controlling phytoplankton alpha‐ and beta‐diversity patterns. Until now, no empirical evidence has been presented to support this hypothesis.Using hydrographic and taxonomic composition data collected near Iceland during the period of the 2008 spring bloom, we show that the distribution of phytoplankton alpha‐ and beta‐diversity was related to submesoscale heterogeneity in oceanographic conditions. Distinct phytoplankton communities as well as differences in richness were identified on either side of a front delimiting surface waters of slightly different (˜0.03) salinities.Alpha‐diversity was significantly higher on the high salinity side of the front compared to the low salinity side. This difference was primarily driven by the presence of several large diatom species in the high salinity region, especially of the genusChaetoceroswhich dominated the biomass here. By investigating beta‐diversity in relation to environmental and spatiotemporal variables, we show that the regional distribution of phytoplankton taxa was influenced by both different environmental conditions on either side of the front and dispersal limitation across the front. Changes in beta‐diversity were primarily driven by turnover rather than nestedness and were apparently controlled by different processes in each region.Synthesis. This study shows that small‐scale and ephemeral density discontinuities created by submesoscale frontal dynamics can play a major role in structuring patterns of phytoplankton diversity. Evidence is presented that they can generate changes in environmental conditions (leading to environmental filtering) and act as physical (dispersal) barriers for phytoplankton transport. The study suggests that dispersal barriers are potentially of much greater importance for phytoplankton diversity at local scales than currently recognized and indicates that drivers of marine phytoplankton diversity are similar to those structuring diversity of land plants.

Size of Dominant Diatom Species Can Alter Their Evenness.

Traditionally, biodiversity has often been estimated on the basis of abundance partly due to the need for complicated measurements of biomass. Here, we conducted robust measurements of the community composition and of the size structure of diatoms in the North Pacific to evaluate the importance of biomass on the biodiversity. We found that the two most useful evenness indices increased in most cases where small species were numerically dominant when calculations were based on biomass compared with those on abundance. Size-abundance spectra of diatoms revealed that numerically dominant small species rarely dominated in terms of biomass. On the other hand, intermediate to large diatom species generally played a dominant role in terms of biomass in diatom community. The results suggest that the size of the dominant species is a crucial factor in determining the role of diatoms in the ecosystem functioning. Because such size variability can also be observed in other organisms, we need to pay attention to the effect of size structures on biodiversity.

Environmental conditions during phytoplankton blooms in the vicinity of James Ross Island, east of the Antarctic Peninsula

In waters surrounding James Ross Island (JRI), which is generally inaccessible, high chlorophyll-a concentration [Chla] can often be detected during summer periods by ocean color imagery. The region is influenced by a retreating sea ice edge from Weddell Sea and freshwater runoff from JRI glaciers, factors that probably trigger phytoplankton growth. In this work, we relate phytoplankton composition and biomass [Chla and carbon] with environmental factors in two successive late summer periods, in 2008 (1–3 March) and 2009 (17–20 February). Remote sensing data were used to corroborate the findings during those few sampling days. High surface [Chla] patches were observed through both remote sensing and field data (up to 7.61 mg Chla m−3 in 2009), and associated with a relatively shallow upper mixed layer (UML) (19–109 m in 2008 and 16–74 m in 2009). Sea surface temperatures were lower in 2008 (−1.19 to −0.62 °C) than in 2009 (−0.87 to −0.46 °C). Sea ice coverage was greater in 2008 than in 2009 summer, implying an earlier sea ice retreat in the latter year, when higher average [Chla] was obtained from field samples (3.3 mg m−3, compared to 1.5 mg m−3 in 2008). The eastern side of JRI appeared to be relatively sheltered from the dominant pattern of large-scale westerly winds. Diatoms dominated the phytoplankton community, with presence of large diatom species (e.g., Odontella weissflogii) typical of an advanced sea ice melt condition. Those blooms were sustained by a shallow UML associated with relative shelter from winds, due to proximity with the island.

Novel basal, fungal lineages from freshwater phytoplankton and lake samples.

Zoosporic fungal parasites are known to control the extent and development of blooms of numerous phytoplankton species. Despite the obvious importance of ecological interactions between parasitic fungi and their phytoplanktonic hosts, their diversity remains largely unknown due to methodological limitations. Here, a method to genetically analyse fungi directly from single, infected colonies of the phytoplanktonic host was applied to field samples of large diatom species from mesotrophic Lake Biwa and eutrophic Lake Inba, Japan. Although previous research on interaction between lacustrine fungi and large phytoplankton has mainly focused on the role of parasitic Chytridiomycota, our results revealed that fungi attached to large diatoms included not only members of Chytridiomycota, but also members of Aphelida, Cryptomycota and yeast. The fungi belonging to Chytridiomycota and Aphelida form novel, basal lineages. Environmental clone libraries also support the occurrence of these lineages in Japanese lakes. The presented method enables us to better characterize individual fungal specimens on phytoplankton, and thus facilitate and improve the investigation of ecological relationships between fungi and phytoplankton in aquatic ecosystems.

Large centric diatoms allocate more cellular nitrogen to photosynthesis to counter slower RUBISCO turnover rates

Diatoms contribute ~40% of primary production in the modern ocean and encompass the largest cell size range of any phytoplankton group. Diatom cell size influences their nutrient uptake, photosynthetic light capture, carbon export efficiency, and growth responses to increasing pCO2. We therefore examined nitrogen resource allocations to the key protein complexes mediating photosynthesis across six marine centric diatoms, spanning 5 orders of magnitude in cell volume, under past, current and predicted future pCO2 levels, in balanced growth under nitrogen repletion. Membrane bound photosynthetic protein concentrations declined with cell volume in parallel with cellular concentrations of total protein, total nitrogen and chlorophyll. Larger diatom species, however, allocated a greater fraction (by 3.5 fold) of their total cellular nitrogen to the soluble RUBISCO carbon fixation complex than did smaller species. Carbon assimilation per unit of RUBISCO large subunit (C RbcL-1 s-1) decreased with cell volume, from ~8 to ~2 C RbcL-1 s-1 from the smallest to the largest cells. Whilst a higher allocation of cellular nitrogen to RUBISCO in larger cells increases the burden upon their nitrogen metabolism, the higher RUBISCO allocation buffers their lower achieved RUBISCO turnover rate to enable larger diatoms to maintain carbon assimilation rates per total protein comparable to small diatoms. Individual species responded to increased pCO2, but cell size effects outweigh pCO2 responses across the diatom species size range examined. In large diatoms a higher nitrogen cost for RUBISCO exacerbates the higher nitrogen requirements associated with light absorption, so the metabolic cost to maintain photosynthesis is a cell size-dependent trait.

No microzooplankton grazing during a Mediopyxis helysia dominated diatom bloom

The new diatom species Mediopyxis helysia was described to science from clones found in 2003 in the North Sea, northern Wadden Sea, and the Gulf of Maine. Seven years after its first occurrence, we observed Mediopyxis to contribute up to almost 50% of the biovolume of the diatoms during a diatom spring bloom in the western Wadden Sea. Grazing experiments based on the dilution technique could not detect any microzooplankton grazing impact on the bloom community. Mediopyxis is now also well established in the western Wadden Sea and does have the potential to become a dominant species. The reasons for its success remain largely unresolved but avoidance of being grazed might be one factor. Future research on this new species is needed to understand the success and forecast the ecological footprint of this large diatom species arriving in the western European Seas.

Size Dependent Competition in Centric Diatoms as a Function of Nitrogen and Silicon Availability

Size dependent competition was examined in two marine centric diatoms, Coscinodiscus sp. and Thalassiosira rotula at various, and Si concentrations. The growth responses for both species to nutrient levels were evaluated using two forms of nitrogen (NH4 and NO3) and silicon in both monoculture and mixed culture conditions. Under single species culture, the impact of Si did not depend on N forms for both diatoms. The increase of NH4-N enhanced the growth of Coscinodiscus, but did not affect T. rotula. When NO3-N was the nitrogen source, cell densities of both species were significantly enhanced by increasing Si concentrations, but only T. rotula density was affected by increasing N concentration. When Coscinodiscus sp. and T. rotula grew in the same culture, Coscinodiscus sp. dominated in both N forms. The scale of the dominance of Coscinodiscus sp. over T. rotula increased with decreasing N and Si concentrations. In the competition experiment, when was the N source, both Coscinodiscus sp. and T. rotula were signifi- cantly affected by changes in N concentration, but only T. rotula was affected by Si. When NO3 was the N source, neither Coscinodiscus sp. nor T. rotula was affected by Si, but T. rotula was enhanced by N levels. Regardless the N form, the impact of Si on neither Coscinodiscus sp. nor T. rotula depended on N concentration. Our results indicate that large diatom species have a competitive advantage over small species, and both large and small species were sensitive to NH4-N limitation, but the small species was more sensitive to NO3-N limitation than the large species.

Responses of microplankton community structure to iron addition in the Scotia Sea

The Southern Ocean is largely a High Nutrient Low Chlorophyll (HNLC) region where macronutrient concentrations are high and phytoplankton productivity is low. However, there are productive ‘hot spots’ that sustain large phytoplankton blooms. These areas, maintained by natural iron (Fe) fertilization, are important for the Southern Ocean ecosystem and for driving carbon export. Fe addition on-deck bioassay experiments were conducted on two cruises to the Scotia Sea region of the Southern Ocean (austral spring 2006 and summer 2008) to better understand how Fe controls the microplankton (20–200 μm) community structure on a seasonal basis. Light microscopy and fast-repetition rate fluorometry were used to examine changes in the species composition and physiological status of the microplankton community. Bioassays were carried out in three contrasting regions of the Scotia Sea: (1) a naturally Fe-fertilized, high chlorophyll area downstream (north and northwest) of the Islands of South Georgia (DSG); (2) a low Fe, low chlorophyll area upstream (south) of the Islands of South Georgia (USG); and (3) a naturally Fe-fertilized area north of the South Orkney Islands (SOI). Multivariate statistics were applied to the light microscopy results, showing significant differences between the initial microplankton communities for each of the bioassays. These differences were primarily spatial (between regions) and secondarily temporal (between seasons). Significant microplankton community shifts occurred in three of five bioassays, those in spring and summer USG and in summer DSG only. In summer, USG community responses increased significantly in medium (100–1000 pg C cell −1) and large (>1000 pg C cell −1) diatom species in response to Fe addition. Such a response was consistent with relief from in situ Fe limitation, which favours larger microplankton species with higher Fe requirements and subject to lower grazing pressures. The largest biomass increase in Fe-treated bioassay bottles was in Pseudonitzschia spp., which suggests that this genus may be a particularly important member of the microplankton community in the Scotia Sea.

Silicate:nitrate ratios of upwelled waters control the phytoplankton community sustained by mesoscale eddies in sub-tropical North Atlantic and Pacific

Abstract. Mesoscale eddies in sub-tropical gyres physically perturb the water column and can introduce macronutrients to the euphotic zone, stimulating a biological response in which phytoplankton communities can become dominated by large phytoplankton. Mesoscale eddies may therefore be important in driving export in oligotrophic regions of the modern ocean. However, the character and magnitude of the biological response sustained by eddies is variable. Here we present data from mesoscale eddies in the Sargasso Sea (Atlantic) and the waters off Hawai'i (Pacific), alongside mesoscale events that affected the Bermuda Atlantic Time-Series Study (BATS) over the past decade. From this analysis, we suggest that the phytoplankton community structure sustained by mesoscale eddies is predetermined by the relative abundance of silicate over nitrate (Si*) in the upwelled waters. We present data that demonstrate that mode-water eddies (MWE) in the Sargasso Sea upwell locally formed waters with relatively high Si* to the euphotic zone, and that cyclonic eddies in the Sargasso Sea introduce waters with relatively low Si*, a signature that originated in the iron-limited Southern Ocean. We propose that this phenomenon can explain the observed dominance of the phytoplankton community by large-diatom species in MWE and by small prokaryotic phytoplankton in cyclonic features. In contrast to the Atlantic, North Pacific Intermediate Water (NPIW) with high Si* may influence the cyclonic eddies in waters off Hawai'i, which also appear capable of sustaining diatom populations. These observations suggest that the structure of phytoplankton communities sustained by eddies may be related to the chemical composition of the upwelled waters in addition to the physical nature of the eddy.

Late summer diatom biomass and community structure on and around the naturally iron-fertilised Kerguelen Plateau in the Southern Ocean

Analysis of the diatom assemblage during the recent KErguelen: compared study of Ocean and Plateau in Surface water (KEOPS) mission (January–February 2005), enabled a modern description of the summer bloom community over the Kerguelen Plateau in the context of the community in the surrounding high-nutrient, low-chlorophyll (HNLC) Southern Ocean waters. Net samples revealed biogeographic partitioning of certain species. Comparison of net samples with CTD-Niskin bottle samples revealed a considerable underestimation of large diatom species in the water samples. We analysed four plateau stations and one off-plateau HNLC station for individual species abundances and biomass contributions down to 150 m. The stations can be divided into two groups based on species composition and total biomass contributions, equating to high (45.6–99.4 μg C L−1) and low (2.5–25.7 μg C L−1) biomass regimes. Individual species abundances were not related to the major species biomass contributions. Repeat analyses at the bloom station, A3, and the off-plateau HNLC station, C11, revealed evolution from a Chaetoceros subgenus Hyalochaete bloom to a remnant Eucampia antarctica assemblage. In contrast the HNLC station, C11, remained dominated by Fragilariopsis pseudonana and F. kerguelensis throughout the survey. Comparison to artificial iron experiments reveals differences in the responses of Pseudo-nitzschia spp. and F. kerguelensis, which may arise in part from differences in macro-nutrient supply, in particular silicic acid availability, as well as from seasonal succession.

Application of AFM in understanding biomineral formation in diatoms

We review previous work and present new data on the application of atomic force microscopy (AFM) to study biomineral formation in diatoms, unicellular algae that make cell walls of silica. Previous studies examined a small subset of mostly larger diatom species, identifying a prevalence of large particulate silica on the nanoscale. We survey different structures including valves, girdle bands, and elongated spines called setae, in a variety of species, and show a diversity of nano- and meso-scale silica morphologies, even on different portions of the same structure. A general trend of highly organized mesoscale silica structure on the proximal face of cell wall components was observed, with less organized structure occurring on the distal face. The highly organized structures have features suggestive of an underlying linear template, which defines the area of initial silica polymerization. Such features have not been imaged with such clarity previously, demonstrating the advantages of AFM to image small differences in surface morphology and providing new insights and confirming evidence for models of diatom silica structure formation. In addition to its imaging capability, more developed application of AFM to map locations of organic template components on the nanoscale will greatly aid in elucidating mechanisms of diatom biosilica synthesis.

Phytoplankton community composition around the Crozet Plateau, with emphasis on diatoms and Phaeocystis

Phytoplankton community composition has important implications for upper-ocean biogeochemistry and export production. The species composition of the phytoplankton community (diatoms, Phaeocystis antarctica) was examined around the Crozet Plateau (Southern Ocean) during the CROZEX project in order to address spatial changes in species composition. Phytoplankton biomass was dominated by P. antarctica to the north of the Crozet Plateau, while south of the plateau a diverse community including several large diatom species (e.g., Corethron pennatum) occurred. The phytoplankton community in the vicinity of the Crozet Plateau and Crozet islands included a mixture of P. antarctica and Thalassionema nitzschioides. High numbers of empty diatom frustules characterised many of the sampling sites, with similar species present in the water column as both full and empty cells. Multivariate analysis was used to analyse both the level of similarity between the different sampling sites and the co-occurrence of species. Cluster analysis of the sampling sites showed distinct clusters that differed in terms of species composition and environmental parameters. Recurrent species analysis showed groupings which relate to phytoplankton cell size; groupings separated (i) small (∼15μm) diatom species, (ii) medium (∼35μm) and large (∼90μm) diatom species as well as P. antarctica colonies, and (iii) giant (∼300μm) diatoms. The abundance and distribution of the different floral groups identified by multivariate analysis are linked to the different environmental regimes, although the relationships are different for small and large species due to the interaction between factors important for phytoplankton growth (e.g., light, nutrients, iron) and mortality (e.g., grazing).

Scaling-up from nutrient physiology to the size-structure of phytoplankton communities

In many community assemblages, the abundance of organisms is a power-law function of organism size. In phytoplankton communities, changes in size structure associated with increases in resource availability and total biomass hate often been interpreted as a release from grazer control. A metapopulation-like approach is used to scale up from the individual physiological responses to environmental conditions to community size structure assuming the community taxonomic composition reflects the species pool. We show that the size scaling of cellular nutrient requirements and growth can cause (1) the power-law relationship between cell size and abundance, (2) dominance of small phytoplankton cells under oligotrophic conditions, and (3) relative increase in abundance of larger phytoplankton cells under eutrophic conditions. If physiological differences associated with the taxonomic composition of different community size fractions are considered, then the model can replicate detailed field observations such as the absence of small, slow-growing Prochlorococcus spp. and the relative dominance of large diatom species in nutrient-rich, upwelling regions of the ocean.

THE ROLE OF THE REACTIVITY AND CONTENT OF IRON OF AEROSOL DUST ON GROWTH RATES OF TWO ANTARCTIC DIATOM SPECIES1

The atmosphere is widely recognized as a major source of Fe in the form of iron‐containing dust. This study provides the first experiments in which the impact of dust on the growth rates of single species of Antarctic diatoms was assessed under laboratory conditions. The dust was among others characterized by x‐ray powder diffraction analysis, analysis of total and amorphous Fe content, and dissolution rates of Fe in seawater. The amount of bioavailable Fe from the dust was determined, not via the complicated chemistry of Fe in seawater but by using diatoms as bioindicators for available Fe. Cultures of two large diatom species, Actinocyclus sp. and Thalassiosira sp., were amended with potential dust aerosols from two dust‐supplying regions, Namibia and Mauritania, and responses on growth rates were monitored. Apart from a difference in total Fe content, a difference in crystallinity existed in the Fe minerals of both dust types. The fraction of amorphous Fe was reflected in a higher reactivity/dissolution of Fe in seawater. The increase in growth rate upon dust addition was positively related with the amount of amorphous Fe in the dust and with the dissolution rate of Fe in seawater. However, compared with equal FeCl3 concentrations, the dissolved Fe from the dust was not completely available for the diatoms. Interestingly, the diatoms used only a small part of the dissolved Fe, demonstrating the importance of algae as bioindicators.

Gradient responses of diatom communities in the Bothnian Sea (northern Baltic Sea), with emphasis on responses to water movement

This study analyses diatom communities on submerged stones in three areas of the Bothnian Sea, a subbasin of the brackish Baltic Sea. The communities were composed of mixtures of epilithic, epiphytic, epipelic, epipsammic and occasionally some pelagic diatoms. Altogether, 120 quantitative epilithic samples, 40 per area, were collected in spring. Exposure to wave action was the major environmental factor varying among sampling sites. Relative ignition loss, used in this study as an indirect measure of macroalgal cover, was positively correlated with exposure to wave action and negatively with dry weight and diatom species diversity. This suggests that macroalgae were more abundant on stones with higher exposure to wave action. The weight of sand grains on the sampled stones was negatively correlated with exposure to wave action. Relative abundance of diatom species was recorded in two separate biovolume classes. For both classes, canonical correspondence analysis revealed that the major environmental factor stru cturing the diatom communities was exposure to wave action. Of secondary importance were differences in community composition explained by nutrient variables. Epipelic species, e.g. Navicula gregaria, N. rhynchotella and Surirella brebissonii, as well as small epipsammic species, e.g. Martyana atomus, Opephora mutabilis and Staurosira punctiformis, were typical of no or low water movement. By contrast, the relative abundance of epiphytes, e.g. Diatoma moniliformis and Tabularia fasciculata, increased with stronger water movement. This was presumably related to the higher availability of macroalgal substrata at sites exposed to wave action. Dominance of epipsammic species separated stagnant water from all other levels of water movement. Large diatom species (≥ 1000 μm3) showed a continuous gradient response to water movement; small species (< 1 000 μm3) gave a “threshold” response.

THE IMPORTANCE OF DIATOM CELL SIZE IN COMMUNITY ANALYSIS1

The large variation in size and shape in diatoms is shown by morphometric measurements of 515 benthic and pelagic diatom species from the Baltic Sea area. The largest mean cell dimension (mostly the apical axis) varied between 4.2 and 653 μm, cell surface area between 55 and 344,000 μm2, and cell volume between 21 and 14.2 × 106μm3. The shape‐related index, length to width ratio, was between 1.0 and 63.3 and the shape‐ and size‐related index, surface area to volume ratio, was between 0.02 and 3.13. Diatom community analysis by multivariate statistics is usually based on counts of a fixed number of diatom valves with species scores irrespective of cell size. This procedure underestimates the large species for two reasons. First, the importance of a species with higher cell volume is usually larger in a community. Second, larger species usually have lower abundances and their occurrence in the diatom counts is stochastic. This article shows that co‐occurring small and large diatom species can respond very differently to environmental constraints. Large epiphytic diatoms responded most to macroalgal host species and small epiphytic diatoms most to environmental conditions at the sampling site. Large epilithic diatoms responded strongly to salinity, whereas small epilithic diatoms did so less clearly. The conclusion is that different scale‐dependent responses are possible within one data set. The results from the test data also show that important ecological information from diatom data can be missed when the large species are neglected or underestimated.

Diatoms provide attachment sites for other diatoms: a natural history of epiphytism from southern California

Large epiphytic and epilithic diatom species were observed to act as substrata for other diatoms at freshwater, brackish and marine sites in southern California, USA. The most commonly encountered examples involved species that form long filaments attached to rocks or macroalgae. Biddulphia biddulphiana, B. tridens, Odontella aurita, Triceratium pentacrinus f. quadrata and Trigonium arcticum were commonly observed hosts in marine sites. Brackish water examples were Bacillaria paxillifer, Melosira varians, Pleurosira laevis and Terpsinoë musica, and Hydrosera whampoensis and P. laevis provided attachment sites in freshwater habitats. These large diatoms had other diatoms attached, usually to the girdle bands or to flat or concave surfaces of the valve face and mantle. Occasionally, the mucilage pads connecting the cells served as substrata for small epiphytic diatoms. The attached flora comprised several unrelated genera of centric and pennate diatoms, both raphid and araphid. Thus, large epiphytic and epilithic diatoms provide suitable attachment sites for other diatom species in water bodies spanning a wide salinity range, and it appears that this phenomenon is more common than previously recognized. The frustule geometry of the ‘host’ diatom apparently plays an important role in determining the location of attachment.